Chemical pulps as they are produced by the chemical processing of wood chips or the like contain a significant amount of lignin, generally about 2 to about 6 wt %. Such pulps are commonly subjected to bleaching and purification operations in a bleach plant, including delignification of the pulp.
Chemical pulps are generally bleached with chlorine-containing bleaching agents, such as, chlorine, chlorine dioxide and sodium hypochlorite. The effluents from such chlorine-based bleaching processes represent a disposal problem, in that they contain chlorinated organics, have a high biochemical oxygen demand and contain chlorides which inhibit integration with the recovery process of the pulp mill without extensive modification thereto.
Typical of such prior art bleaching operations are those described in U.S. Pat. No. 2,882,965 to Wayman et al wherein there is described the treatment of wood with chlorine, a high temperature caustic extraction and subsequently with chlorine dioxide. A substantial amount of the lignin is removed during the initial chlorine treatment stage and the remaining chlorinated lignin then is removed by caustic extractions.
Hydrogen peroxide is theoretically capable of decreasing the disposal problems posed by chlorine-containing bleaching agents since no chlorinated organics nor chlorides are formed so that the peroxide stage effluent may be taken into the pulping chemical recovery cycle and there destroyed and thus the biochemical oxygen demand is lower. Despite these attractions, hydrogen peroxide has rarely been used, mainly because of its historically high cost. Such cost, however, has been decreasing of late, to the point where there is an interest in using hydrogen peroxide as a substitute for chlorine-based bleaching compounds in the intermediate bleaching stages. However, interest in the utilization of hydrogen peroxide in delignification during the first stage of bleaching has been very small, mainly because oxygen bleaching is much more economical.
One prior art proposal to use hydrogen peroxide delignification involves an initial treatment of the pulp with alkaline hydrogen peroxide to effect partial delignification of pulp prior to bleaching to high brightness with a conventional CEDED sequence using less chlorine. This procedure is the so-called "MINOX" process, described in Tappi Pulping Conference, Preprints (1980), p.325.